The present invention relates to a color cathode ray tube.
In general, a color cathode ray tube comprises an envelope including a substantially rectangular panel provided with a skirt portion at the periphery of the panel, and a funnel. On the inner surface of the panel is formed a phosphor screen which includes a number of phosphor layers of three colors which radiate in red, blue,.and green. In the neck of the funnel is arranged an electron gun for emitting electron beams toward the phosphor screen. Inside the phosphor screen, a shadow mask is provided and opposed to the phosphor screen with a predetermined distance maintained therebetween.
In the color cathode ray tube, electron beams emitted from the electron gun are deflected by a deflector and subjected to selection by the shadow mask, so that the phosphor screen is scanned horizontally and vertically by the electron beams to display a color image.
The shadow mask has a substantially rectangular mask body having a surface opposed to the phosphor screen where a number of electron beam apertures are formed, and a rectangular mask frame welded to the periphery of the mask body. Plate-like frame holders are welded to the side walls of the mask frame. The shadow mask is supported on the inside of the panel by engaging the frame holders with stud pins fixed to the skirt portion of the panel.
In many of structures used for installing the mask body on the mask frame, the mask body is welded to the mask frame at each corner and at one or plural points in the area of the center of each side edge.
As a support structure for supporting the shadow mask on the panel, there has been a structure in which the shadow mask is supported by a band-like frame holder welded to the substantial center of each side wall. In this kind of shadow mask and the support structure thereof, the mask body and the mask frame are generally welded to each other, at positions which are slightly distant from engaging points of stud pins of the frame holder, avoiding welding points between the mask frame and the mask holder.
Meanwhile, 30% or less of the entire electron beams emitted from the electron gun enter into the phosphor screen, and the rest of the electron beams collide to the shadow mask. The kinetic energy of those electron beams is converted into thermal energy which heats the shadow mask and frame holder. If the shadow mask is thermally expanded by the heat, electron beam spots shaped by the shadow mask and formed on the phosphor screen are shifted from predetermined three-color phosphor layers, i.e., electron beams cause miss landing in relation to the phosphor layers. As a result, color purity is deteriorated.
This kind of miss landing of electron beams is roughly divided into two cases. In one case, miss lading is caused mainly by the mask body heated and thermally expanded, in an early stage of operation after the color cathode ray tube is started. In the other case, miss landing of electron beams is caused by the mask frame or the frame holder thermally expanded due to heat transferred from the mask body during operation of the color cathode ray tube for a long time (i.e., long-term purity drift).
In a certain cathode ray tube, a mask body made of invar (iron-nickel alloy) having a low thermal expansion coefficient is used in place of a mask body made of soft steel, in order to reduce the miss landing of electron beams caused by the thermal expansion of the shadow mask. In this case, the thermal expansion of the mask body itself can be reduced to be small.
However, when both the mask frame and the frame holders are thermally expanded during operation for a long time, there occurs a phenomenon that the mask body causes a localized displacement in asymmetric directions and landing of electron beams relative to the three-color phosphor layers is misregistered in asymmetric direction, e.g., in vertical, lateral, and rotational directions.
If a mask frame is not provided, a heated mask body is thermally expanded symmetrically in the radial direction and landing of electron beams is therefore not misregistered in asymmetric directions such as vertical, lateral, and rotational directions. Hence, misregistration in asymmetric directions such as vertical, lateral, and rotational directions is estimated to occur depending on the structure of installing the mask frame and the support structure of the shadow mask with respect to the mask body.
Taken into consideration a shadow mask having a mask body made of invar, a mask frame made of soft steel, and a frame holder made of stainless steel, the thermal expansion of the mask body is as small as 1/10 of that of the mask frame. Therefore, if the shadow mask is thermally expanded, the mask body is tensioned outwardly. However, the thermal expansion of the mask frame is absorbed by the skirt portion of the mask body, and does not substantially make effects on the effective portion of the mask body.
However, due to the thermal expansion of the mask frame, for example, welding points of the mask body on the short side edges thereof are shifted in the vertical direction (or Y-axis direction), and as a result, peripheral portions of the short side edges of the mask body are shifted in the vertical direction. Likewise, peripheral portions of the long edges of the mask body are shifted in the lateral direction (or X-axis direction) due to the thermal expansion of the mask frame. Because of these shifts in both the vertical and lateral directions, the peripheral portions of the mask body are shifted, as a whole, in the rotational direction.
Therefore, in the color cathode ray tube described above, landing of electron beams relative to the three-color phosphor layers is misregistered in asymmetrical directions such as the vertical, lateral, and rotational direction, so that white uniformity is deteriorated.
Among thermal expansions of the mask frame and mask holder, deterioration of the color purity caused by shifts of beam spots due to the thermal expansion of the mask frame can be reduced, to some extent, by adjusting electron beams in the inner circumferential direction of the screen, assuming a high luminance condition, in a stage of preparing factory preset or the like. However, as for deterioration of the color purity due to operation of the mask holder under a high luminance condition, electron beams cannot be adjusted in the rotational direction in the stage of preparing factory preset due to the structure of the color cathode ray tube, and therefore, deterioration of the color purity in the rotational direction cannot be reduced by any previous adjustment.